1. Field of the Invention
This invention relates to a technique for the fabrication of a mask destined for use in resist and lithographic processes. More particularly, the present invention relates to technique for the fabrication of iron oxide photomasks utilizing chemical vapor deposition techniques.
2. Description of the Prior Art
The need for processing localized areas in microcircuit technology has generated a technology directed to the efficacious preparation and utilization of masks to define diffusion, evaporation and related operations. The use of well-known photomask processes for attaining this end has been universally applied in the microcircuit processing industry with varying degrees of success. Typically, such techniques involve the preparation of a suitable photomask defining the pattern of interest and the use of this mask to transfer an image to a photoresist pattern.
Until recently, it had been conventional to form the mask pattern in a photographic emulsion. In numerous applications, the masks so formed are used repetitively and due to the inherent softness of photographic emulsions deteriorate rapidly due to abrasion. Accordingly, workers in the art turned their attention toward the development of masks manifesting greater durability.
This end was attained by the use of hard inorganic opaque materials, typically metal on glass. A popular mask falling within the scope of this class is prepared by evaporating chromium upon a glass substrate and thereafter forming the desired pattern in photoresist on this surface. Thereafter, the pattern is etched into the chromium. These masks have been found to be very durable and manifest a potentially higher resolution capability than photographic emulsion masks, such being attributed both to the thinness of the deposited metal and the lack of grain and thinness in the image defining photoresist. Although satisfactory from many standpoints, such photomasks are opaque and reflect a high percentage of incident radiation including the light normally used during alignment of the photomask with respect to pattern previously imposed on the substrate. Both the opacity and reflectivity contribute to the difficulty of carrying out this alignment, especially on equally reflective metallized substrates. These masks also reflect the light normally used to expose the photoresist after alignment, so creating a problem of fringing with the concomitant loss of resolution at the edges of the pattern due to multiple reflections between the substrate and the photomask.
Reactively sputtering a hard inorganic compound upon a glass substrate and etching the deposited layer to form the desired pattern represented a suggested approach for improved masks. Some transition metal oxides were found promising in being transparent to the light used by the operator to align the photomask with the substrate and highly absorbing at the wavelength used to expose the photoresist on the substrate to be processed. But iron oxide films which were produced in this manner and which had these desirable optical properties were, to the date of the present invention, considered unsuitable for patterning into masks by the usual photoresist processing because of their relative insolubility in the usual etchants compatible with the photoresists.